1. Field of the Invention
The present invention generally relates to charged particle beam systems employing a shaped beam of charged particles and, more particularly, to electron beam lithography tools having a beam-shaping aperture.
2. Description of the Prior Art
Many lithographic processes are well-known and highly developed, particularly for semiconductor integrated circuit manufacturing processes. Electromagnetic radiation has generally been the exposure medium of choice for exposing a resist which is later developed to form a pattern which will define the location and basic dimensions of devices which are included in the integrated circuit. However, the need for increased performance and functionality of integrated circuit chips and increased economy of manufacture has led to design rules and feature size regimes of 0.1 micron or smaller which cannot be produced even with very short wavelength radiation for which suitable resists are known. Accordingly, charged particle beams have been used for resist exposures to develop feature sizes in such regimes.
An electron beam is generally preferred to beams of other charged particles for exposure of a resist since the much lower mass of electrons relative to ions allows the beam to be readily controlled. It is a relatively common practice in production of extremely fine features to provide shaping of the beam with a shaped aperture or respective portions or sub-fields of a patterned mask to build up the desired exposure pattern through multiple exposures. Accurate shaping of the beam is extremely important since it controls the quality of the exposure spot.
Use of an electron beam is also preferred to ion beams since it also limits the deposition of material on the interior of the tool and structures which intercept portions of the beam, in particular. Nevertheless, some deposition of contaminants is unavoidable in electron beam systems and is particularly severe where the electron beam must impinge upon some structure, such as where a shaped aperture is provided in a plate in order to shape the electron beam. The contaminant materials are principally cracked hydrocarbons and silicon compounds produced by decomposition (due to the energy of the beam) of lubricants/sealants required for assembly and proper sealing of the vacuum chamber of the tool and outgassed from the exposed imaging resist.
By the same token, the position of the effective edges of the aperture or other structure is particularly critical to the production of feature sizes for which the electron beam tool is required and the effective position of the edges is affected by deposits thereon. Further, deposits of cracked hydrocarbons and silicon compounds exhibit low conductivity and may thus accumulate charge thereon which can serve to further distort or even deflect the beam; compromising beam shape and position of impingement on the target (e.g. resist on a wafer).
The throughput of the electron beam tool is also critical to manufacturing efficiency. Therefore, manufacturing efficiency is severely compromised by periods of service and reduced useful production time between periods of service or even recalibration. When deposits of contaminants compromise the accuracy (e.g. shape and/or location) of the lithographic pattern that can be produced, there is no alternative to removing the tool from useful service to renew or replace the parts on which deposits of contaminants have formed. Beam shaping aperture plates are expensive to produce and are very delicate, generally requiring replacement and increasing the cost of operating the electron beam tool.
In this regard, it is known that an elevated surface temperature of an element (equal to or greater than 200.degree. C.) reduces the rate of deposition of contaminants on that element and can thus increase the intervals between required periods for servicing of the tool and reduce the number of new or renewed aperture plates required for a given number of exposures. To achieve such elevated temperatures, it has been the practice, in tools using Gaussian beams (e.g. having an electron density or flux distribution across the beam which is approximately Gaussian) to make the aperture plate self-heating since the aperture absorbs the energy in the "tails" of the Gaussian distribution of electron flux to improve beam current uniformity across the remaining beam.
Self-heating is generally accomplished by thermally isolating the aperture foil (used to conduct the intercepted beam current) from the thermal mass of the beam column (i.e. supporting apparatus for the aperture plate being formed of webs, stand-offs or the like, preferably of materials of low thermal conductivity). By limiting conduction of heat derived from absorbed electron energy away from the aperture foil, an elevated temperature can be achieved.
However, this solution is less than fully satisfactory since the heating may be irregular if the beam is not exactly centered on the aperture and uneven heating (and consequent uneven expansion of the aperture plate structure with increased temperature) tends to shift the aperture away from the region where the greatest beam current is incident, aggravating the uneven heating even if an approximately constant aperture foil temperature can be achieved. For shaped beam systems, self-heating is impractical since the absorbed electron current and energy is necessarily dependent on the amount and nature of the beam shaping which fluctuates over the course of pattern writing and prevents a constant aperture foil temperature from even being approximated.
Another attempt to provide elevated temperature of the aperture foil is to provide a separate heater mounted close to or in contact with the aperture foil. However, such an expedient complicates the structure of the e-beam tool. Further, such heaters are very delicate and can be easily damaged by routine handling during installation. Moreover, conventional manufacturing techniques for such separate heater elements employ a resistive paste on a ceramic substrate. Manufacture at the dimensions and with the material constraints imposed by the electron beam column have proven to be of low yield and the heater elements have proven to be inconsistently reliable. If such a heater element fails in such an arrangement during a production run, there is a trade-off between rate of deposition of contaminants resulting in acceleration of damage to the aperture, loss of throughput during replacement, and possible reduction of manufacturing yield as the aperture shape and dimensional stability of the beam column become largely unpredictable.